E.D.
My suggestion is to let yourself grieve. It's okay to feel sad that your son has received a (preliminary) diagnosis, it's not what you were expecting, is startling, could feel scary/unknown/new/challenging. That's all a really, really normal reaction.
That being said. A diagnosis will not change your son. He won't be any different than the boy you love and know. The difference is that now you will have more access to tools and resources that will *hopefully* help him (and your family) flourish. A diagnosis is not, in my mind, a barrier. Rather, it is the acknowledgment and acceptance of a state of being. Thus, limitations can be recognized and then worked around. Support can be found. Parenting methods can be utilized. Learning structures specialized.
Thing is, having a neurodisorder in a neurotypical world can be a bit of a challenge. But *knowing* that THAT is what is challenging, can be really, really, liberating. It can help us to be better parents, and to live more effectively, with less struggle, and more fully as children AND adults w/ disorders.
Nonetheless, I hear you. It's tough news to receive and your reaction is totally normal.
Good luck and big hugs.